1. Field of the Invention
The present invention relates to a process for making a catalyst suitable for direct coal liquefaction and the catalysts thereof.
2. Description of Related Art
It is known that direct coal liquefaction (DCL) was developed as a commercial process in Germany, based on the research of Friedrich-Bergius who established that the coal liquefaction consists into two competitive reactions: a) hydrogen addition at high temperature and pressure to turn coal structure into a heavy hydrocarbon, and b) rupture of the heavy hydrocarbons. In this way, the coal liquefaction is a process that converts the carbonaceous material, such as coal, into liquid fuels or “synthetic fuel”. Despite of the different variables of the process, all of them agree that the coal is dissolved in a solvent at high pressure and temperature, and then, a coal-derived liquid fuel is obtained through the coal hydrogenation in the presence of a catalyst.
It is known that the coal liquefaction process is carried out in a reactor at high pressure and temperature using hydrogen, a solvent and a particulate catalyst, obtaining solid residues, synthetic crude oil and gas fuel. The scarcity of light petroleum fractions in the last decades has made the need of processing the heavy crude oil, as well as the transformation of coal, toward the obtaining of liquid fuels. The aim is to exploit this resource and provide a viable option to obtaining alternative energetic products.
Although many research about coal liquefaction has been made, different authors, (STOHL F., DIEGERT K. V. (1994) Development of standard direct coal liquefaction activity tests for fine particle size, iron based catalysts. Energy & Fuels 8, 117-123; PRIYANTO U., SAKANISHI K., OKUMA O., SUMBOGO MURTI S., WATANABLE I., KORAI Y, MOCHIDA I. (2001) Optimization of two-stage liquefaction of Tanito Harem coal with FeNi catalyst supported on carbon black Energy & Fuels 15, 856-862; RINCÓN J. M., ANGULO R. (1986) Petroleum heavy oil mixtures as a source of hydrogen in the liquefaction of Cerrejon coal. Fuel 65 (7) 899-902) have found that the optimum conditions to achieve high effectiveness, strongly depends on reaction temperature, coal type, solvent and catalyst. Since the catalyst strongly affects the coal liquefaction, it has been a high interest in the development of new catalyst, having high activity and selectivity, low cost and environmental friendly.
Gözmen B. and coworkers (2002. Direct liquefaction of high-sulfur coals. Energy & Fuels 16, 1040-1047) have found that the catalyst used in coal liquefaction must be effective in the hydrocracking reactions toward quenching free radicals formed during the thermolysis reactions, removing heteroatoms present in the coal structure, and hydrogenating the appropriate sites in the coal and molecules of the solvent.
It is known that for the coal liquefaction process, a well dispersed catalyst on the support is preferred, because this allows a more intimate contact with the coal. The shape, the conditions and the parameters for catalyst making are important since they determine its properties. By knowing the catalyst properties, the reaction can be controlled, promoting the initial dissolution of coal and reducing unwanted reactions.
It is known that in the coal liquefaction process, a catalyst is used to facilitate the rupture of the coal structure and the hydrogenation of its structural fragments. In the presence of a catalyst, whose active phase can be molybdenum, iron, cobalt, nickel and/or tungsten sulfurs, the reactions of hydrogen transferred are promoted. During the hydrogen transfer, the molecular hydrogen is dissociated into atoms of active hydrogen that stabilize the radicals performing the hydrogenolysis or hydrocracking. This is one of the most critical functions of the catalyst in the initial phase of the liquefaction altogether with the avoiding of retrogressive reactions which lead to the formation of residues of high molecular weight.
It is known that use of bimetallic catalyst in the coal liquefaction process is promising since it has been demonstrated the synergetic effect of the Fe—Mo system. In addition, the state-of-the-art reveals studies where the iron based materials highly dispersed and ultrafine are preferred. The catalyst is made by impregnation in-situ of iron sulfur obtained from mixing a solution of Na2S with FeCl3 or Fe2(SO4)3; a precipitated of iron sulfur on the support is produced. Sometimes, a pretreatment on the surface is carried out with a surfactant in order to enhance the dispersion of the catalyst.
An example was found in the U.S. Pat. No. 5,168,088 where an iron-containing catalyst finely divided or a catalyst precursor is adsorbed on the surface of a particulate coal in the initial steps of direct coal liquefaction or coal hydrogenation. The method consists in precipitate or deposit a catalyst, which is a hydrated iron oxide, directly onto the surface of the coal from a wet paste mixture. Within the precursors of the active form of the catalyst, besides FeOOH, it is mentioned the insoluble iron sulfur such as pyrite. At high temperature, required in the process, the precursor of the catalyst (FeOOH) reacts with H2S or CS2, turning Fe into an active form of pyrrhotite, Fe1-xS where x is in between 0.1 and 0.2. Although coal contains the necessary sulfur to form H2S or CS2, additional sulfur may be required to fully activate the iron catalyst.
The U.S. Pat. No. 6,258,259 enclosed a material of iron sulfide of high purity finely divided and process for producing the same. In this invention it has been found that some synthesized compounds, where FeS2 is the main component, show excellent catalytic activity in the hydrogenation processes, specially when they are used as catalyst for coal liquefaction. The material of iron sulfide has the composition of 85% to 100% in weight of FeS2, 0 to 5% in weight of Fe1-xS where x is 0 to 0.2; from 0 to 5% in weight of Fe3O4 and from 0 to 10% of FeSO4. The process to obtain the material consists in the heating of ferrous sulfate monohydrated and a sulfur compound in an amount not less that the stoichiometric amount in a fluidized bed, being the atmosphere air. The components are burnt and react between 623 and 903K at a pressure of 101,33 kPa.
The U.S. Pat. No. 4,441,983 describes a process for the liquefaction of a solid carbonaceous material at a high pressure and temperature in the presence of a solvent (for the carbonaceous material), hydrogen and a hydrogenation catalyst to produce mainly liquid products, where the improvement comprise the use of an activated zinc sulfide hydrogenation catalyst. Such activation is carried out by exposing the preformed zinc sulfur under hydrogen atmosphere and 755K in a solvent with the absence of the carbonaceous material. The activation is performed in the presence of other sulfurs to avoid the reduction of zinc sulfur during the activation.
Most of the catalysts used in the direct liquefaction contain metallic sulfurs, such as pyrite, which is very active in the hydrogenation process since favor the transference of gaseous hydrogen to the carbonaceous matrix. Additionally, in the DCL, iron-based conventional catalyst are used, which are supported on alumina, zeolites, or activated carbon among others; subsequently, a second metal is incorporated, such as nickel, cobalt, tungsten and molybdenum as catalyst promoters. In spite of all the existent techniques for the production of liquefaction catalysts, there is a great interest in the development of new catalysts highly active and selective, of low cost and environmental friendly.
The catalytic activity has been attributed to the improvement of the dispersion of the catalyst on the coal surface. A highly disperse catalyst provides an effective contact with the solvent and coal at lower concentration of the active phase, reducing the diffusion limitation between particles. Besides, it provides an effective contact with the liquid products coming from the coal liquefaction. The use of hydrogen sulfide increases the yield of liquids in the coal liquefaction process.
It is known that traditionally, the sulfurs of transition metals are obtained when a precursor salt of such metal is reacted with a source of sulfur that can be selected from CS2, H2S, organic compounds that contain sulfurs such as mercaptans or sulfurs obtained under low temperature and pressure conditions. The activation of these catalysts precursors is carried out in the presence of hydrogen at high pressures, from 5 and 10 MPa, and temperatures, from 523 and 753 K.
Because of the severity in the conditions of pressure and temperature to obtain the metal sulfur, this process is expensive and not environmental friendly, since the conditions of handling of the sulfur compounds, and more specifically, the utilization of gases such as CS2, H2S needs extra-handling care. It is well known that H2S, which is the most used in the process, is a colorless inflammable gas, corrosive and that can be poisonous at high concentrations; hence, it needs a high handling security in the production plant that increase the cost in the obtaining of the catalysts.
It is important to highlight that enormous efforts are being made to provide a process for the catalyst preparation of low cost, to make the catalysts marketable. An option to reduce cost are the use of processes that do not use corrosive substances such as H2S or the ones that improve the liquid yields, the quality of the distillates, the activity, selectivity and stability of the catalyst, the amount of solvent required and the use of lower severity reaction conditions.